1. Field of the Invention
The present invention relates to a linear light source/collector, and more particularly, to such a light source/collector for use in film scanners.
2. Description of the Prior Art
In film digitizers, linear light sources and collectors have been frequently employed. A telecine film scanner typically includes a linear CCD image sensor which provides a serial output of signals representing successive lines of an image illuminated by a linear light source. For color applications, the film scanner can include an assembly of three separate CCD image sensors, one for each of the primary colors. The film is driven at a uniform rate past a linear light source, and an illuminated line of the film is imaged onto each CCD image sensor. The film motion provides the frame scan, and the linear cycling of the elements in the image sensor provides the line scan. A scanner of this type is disclosed in U.S. Pat. No. 4,205,337.
The lamps used in such systems normally produce a circularly symmetric light beam, and there is a problem in these systems in providing for an efficient conversion of the circular beam to a uniform line distribution of the light. In U.S. Pat. No. 4,797,711, for example, there is disclosed a scanner in which a transparent cylindrical rod is arranged to produce a line of light on an original. Light from a light source passes through color filters and is then directed onto one end of the cylindrical rod. Light reflected from the original is imaged onto a CCD image sensor. One of the main problems of the illumination system shown in this patent is that it does not provide a uniform line of diffuse light to an original such as film, and thus, artifacts on the original, e.g., scratches, will appear in an image produced from the scanned information.
An improved type of illumination system is disclosed in U.S. Pat. No. 4,868,383, which is assigned to the assignee of the present invention. This patent discloses a linear light source for a film scanner which includes means for generating an intense beam of light and an elongated cylindrical integrating cavity having diffusely reflective walls. The intense beam is introduced into the cavity through an input port, and a uniform line of light is emitted through an output slit which extends parallel to the longitudinal axis of the cylindrical integrating cavity. Such a light source produces a line of diffuse light which has a uniform linear and angular distribution, and excellent results can be obtained over a wide range of operating conditions. There is a need, however, in certain types of scanners for a linear light source in which the linear distribution issuing from such a light source can be very closely controlled in order to control the intensity of the light on a receiving medium.
Other considerations taken into account in the design of telecine scanners and linear light sources are set forth in the articles: J. R. Milch, "Line Illumination System and Detector for Film Digitization," SPIE Proc., Vol. 1242, pp. 66-77, February 1990; M. Kaplan, "Monte Carlo Calculation of Light Distribution in an Integrating Cavity Illuminator," SPIE Proc., Vol. 1448, pp. 206-217, February 1991; and A. F. Kurtz and David Kessler, "Optical Scanning System for a CCD Telecine for HDTV," SPIE Proc., Vol. 1448, pp. 191-205, February 1991.
These light integration cylinders are also used in the inverse fashion to collect light entering the elongated slit in the cylinder wall and present it to a photodetector positioned at an exit port as depicted in FIG. 4 of the above referenced Kaplan article. In this context the slit of the integrating cylinder is oriented to receive light transmitted through (or reflected by) an image leaving medium, e.g. radiographic film, that is scanned linearly by a laser beam. In such scanners the images, recorded in the form of variations in the film density or opacity, are scanned repeatedly with the laser light beam and the transmitted (or reflected) intensity of the beam is continuously read out by the photodetector. The integrating cylinder thus acts to collect the light of the beam entering the slit and direct it to the photodetector. In this context it is important that the integrating cylinder light collector does not, through its design, accentuate or alter the image modulated intensity of the light at the port depending on where it enters the slit.
In the design of such linear light sources and collectors, it has been proposed to employ a light pipe or glass rod positioned within the integrating cylinder and extending parallel to the exit slit in the cylinder wall to distribute light from a light source to the cylinder or to collect light from the cylinder and direct it to the photodetector. Japanese Laid Open Patent Publication Nos. 1-116630 (Application No. 62-275247) 1-143937 (Application No. 62-143937) disclosed such an arrangement where the light pipe has a stripe of barium sulfate to a given width and in the lengthwise direction to form a light scattering band. Light entering one end of the light pipe is scattered, by striking the stripe, into the cylinder and scattered until it exits the elongated stripe. Conversely, light may be collected that enters the slit and directed out the end of the light pipe.
Despite these improvements made in integrating cylinder linear light sources and collectors, difficulties remain in achieving efficiency and uniformity in light intensity emitted or detected along the length of the slit.
The current technology has several limitations which are addressed by the present invention. Integrating cylinders are inefficient, typically wasting 70-90% of the light entering the entry hole. The ideal output profile has somewhat more light at the ends than at the center of the slit, but the current cylinders tend toward the converse, and customizing the profile is difficult or impossible. The source lamp may need to be carefully aligned with respect to the entry hole to produce near optimum results.
As described in certain of the referenced patents, it has also been proposed to provide an integrating cylinder with the entry hole in an end face, rather than on the side of the cylinder. The primary advantage of an end hole is that it mechanically fits better into some scanner systems. A great disadvantage is that the source lamp must be carefully aligned with respect to the cavity. Also, the uniformity of light diffusion in the cavity with the end face entry hole is usually worse than the side entry hole.